1. Field of the Invention
The present invention relates to the recovery of precious metals such as platinum, palladium, and rhodium, and optionally of other valuable elements, such as the rare earths and in particular cerium, from various compositions/materials comprised thereof, especially waste or spent catalysts, for example motor vehicle postcombustion catalysts.
2. Description of the Prior Art
It is known to this art that precious metals such as platinum, rhodium, palladium or iridium are currently widely used, whether alone or in combination, in a variety of industrial compositions, in particular catalyst compositions, and this optionally in conjunction with other valuable elements, such as, rare earths, for example, cerium.
Particularly exemplary such compositions are the so-called multifunctional catalysts (or xe2x80x9cthree-wayxe2x80x9d catalysts), and especially the catalysts for the treatment of the exhaust gases of internal combustion engines (motor vehicles or otherwise). Essentially for environmental protection reasons, considerable research and development in respect of such catalysts is presently ongoing (imposition of increasingly strict antipollution standards vis-a-vis the reduction of deleterious emissions of nitrogen oxides, of carbon monoxide and, equally, of unburned hydrocarbons).
Thus, at present, nearly 80% of the rhodium and 40% of the platinum values which are consumed worldwide are actually employed for the preparation of catalysts destined for the motor vehicle postcombustion sector. In view of the proportionately very high cost generated merely by the use of these precious metals in relation to the total cost of the finished catalysts, the recovery of same from a spent or waste catalyst for purposes of downstream recycling for the production of a new catalyst is a major desideratum in this art.
Processes for the recovery of the precious metals present in spent catalysts are described in the literature.
Particularly representative thereof are those which are principally based on a stage of acidic leaching of the spent catalysts containing the species to be recovered, this stage typically being carried out in the presence of an oxidizing agent.
Even more particularly according to these processes, the catalysts to be treated, which may either be in the form of pellets (beads, extrudates or the like) generally consisting of alumina, in particular of xcex3 active alumina, containing the various precious metals (as well as, optionally, other valuable elements such as, for example, cerium), or in the form of monoliths consisting of a framework (or support) of the honeycomb type made of refractory ceramic, and in most cases based on cordierite (silicoaluminate) onto which a washcoat (or coating layer) of alumina, in particular of xcex3 active alumina, is deposited, and also containing the precious metals (and other valuable elements), are preground and converted into a finely divided state. The powder thus obtained is then leached, one or more times, with strong inorganic acids such as sulfuric acid, hydrochloric acid or nitric acid, optionally in the presence of an oxidizing agent (H2O2, Cl2, HNO3 or other), whereby one or more leaching solutions are obtained containing the desired different precious metals in a dissolved form, these being subsequently recovered from such solutions by any technique which is appropriate and per se known to the art, in particular by selective precipitation.
However, processes such as those described above especially present the disadvantage of exhibiting solubilization, and hence recovery, yields in respect of the various desired precious metals (Pt, Pd, Rh and others) which remain insufficient, this being particularly more so in respect of the rhodium species. Furthermore, other than the precious metals, these processes only very rarely address the case of the recovery of the other valuable elements which may be present in the spent catalysts, such as, for example, the rare earths and, in particular, cerium.
Accordingly, a major object of the present invention is the provision of an improved process for the recovery, at very high efficiency, of the different precious metals, and in particular rhodium, which are present in various compositions, for example of waste or spent catalysts, as well as for the recovery of other valuable elements which may be present in said compositions, such as, for example, the rare earths and, in particular, cerium.
Briefly, the present invention features an improved process for the treatment of any composition or material containing species of the precious metal type and, optionally, rare earths such as cerium, for the recovery of such species, said process including the following stages:
(i) optionally, first converting the composition to be treated into a finely divided state to increase the reactivity thereof,
(ii) intimately admixing this composition with sulfuric acid,
(iii) calcining the mixture thus obtained at a temperature of from 150xc2x0 C. to 450xc2x0 C., and
(iv) finally, treating the product thus calcined, in aqueous medium, on the one hand, with H+ ions and, on the other, with chloride ions, this being carried out simultaneously or separately, whereby there are ultimately obtained, first, a solid residue substantially exhausted in respect of precious metals and, optionally, of rare earths such as cerium, and secondarily, one or more solutions containing the aforesaid species.
More particularly according to the present invention, the combination of the above stages (ii) and (iii) corresponds to an operation which, for simplicity, will subsequently often be deemed xe2x80x9csulfuric pasting.xe2x80x9d
The stage (iv) can be carried out essentially according to two different embodiments: (1) either, this being to selectively recover certain of the desired species, subjecting the calcined product to a double leaching by treating the product, first, with an acidic solution (introduction of the H+ ions) and then, in a second step, with a solution containing chloride ions, whereby there are obtained, respectively, a first solution containing, for example, most of the cerium and a proportion of the rhodium, and a second solution containing, for example, most of the platinum and the remaining proportion of the rhodium; or (2) an overall recovery of the desired species is carried out by leaching all of these species simultaneously by treating the calcined product with a single solution having, on the one hand, the necessary acidity (introduction of the H+ ions) and containing, on the other, the chloride ions.
Lastly, in either of the above alternative embodiments, the treatment with chloride ions is preferably carried out in the presence of an oxidizing agent.
The process according to the invention thus permits a selective or overall recovery, this being with very high efficiencies, of all of the species of the precious metal type or other valuable elements, in particular rhodium and cerium, which are present in the starting composition. It can reasonably be considered that the exceptionally high efficiencies thus obtained are essentially due to the preliminary sulfuric pasting stage which facilitates the subsequent leaching of the desired species.
According to the present invention, by the expression xe2x80x9cprecious metalsxe2x80x9d are intended, whether alone or in combination, all of the elements of the platinum group, namely the elements rhodium, palladium, iridium, platinum, ruthenium and osmium. Furthermore, by the expression xe2x80x9cvaluable elementxe2x80x9d are essentially intended the rare earths, namely the elements of the lanthanide series whose atomic numbers range from 57 to 71, inclusive, and more particularly cerium of atomic number 58, as well as yttrium of atomic number 39. Finally, by the expression xe2x80x9ccompositionxe2x80x9d are intended any composition containing one or more precious metals, optionally in combination with one or more valuable elements, and from which the recovery and subsequently the recycling of said species is intended.
Exemplary compositions that are treated via the process according to the invention include, more particularly, the waste or spent motor vehicle postcombustion catalysts, as well as the corresponding new catalysts when the latter comprise, for example, manufacturing scrap. Taking account of the fact that the process according to the invention is particularly suitable for the combined recovery of the precious metals and of the cerium valves which are present in such catalysts, and that this is a preferred embodiment thereof, the description which follows will be essentially delimited thereto.
The starting catalysts (which therefore contain the species to be recovered, such as precious metals and cerium) may be in various forms, namely, for example, and as indicated above, in the form of pellets or in the form of monoliths. It will be appreciated that such catalysts are widely described in the literature, both in respect of their structure and their composition, and including many patents and/or patent applications.
Thus, these catalysts are generally either essentially based on alumina (in the case of pellets) or, in the more particular case of the monoliths, essentially based on a refractory ceramic such as a silicoaluminate (for example cordierite) in the support phase and based on alumina in the washcoat phase. However, the treatment of a catalyst of monolith type in which the honeycomb (or support) is fabricated not of a conventional ceramic, but of a metal or of a metal alloy, is obviously also within the ambit of the invention; for this purpose, it suffices to first merely extract the washcoat from the support using appropriate mechanical and/or chemical means, and then to treat this washcoat according to the invention, to recover its precious metals or cerium values therefrom.
With catalysts of spent or waste type, it will be appreciated that the latter generally contain various impurities, such as lead, carbonaceous substances and the like. However, the process of the invention is insensitive to the presence of these impurities.
The process according to the invention is particularly suited for the combined recovery of the precious metals and of the cerium values which are present in spent catalysts of the monolith type, comprising a support substrate of cordierite coated with a washcoat of alumina.
According to the first optional stage of the process according to the invention, grinding of the starting catalyst is first carried out. Although this stage is not truly essential, it is nevertheless preferred, in particular in the case of a catalyst which is in a form of a monolith. The catalyst is thus converted into a finely divided state, in particular into the form of a powder whose mean particle size may range, for example from a few microns to a few millimeters. This state enables preparation of a product which is, on the one hand, chemically more homogeneous and, on the other, more reactive in respect of the subsequent process parameters according to the invention.
In another optional embodiment of the process of the invention, the starting catalyst, optionally after grinding, may next be subjected to a thermal pretreatment (calcination) conducted under a reducing atmosphere, for example a mixture of argon and hydrogen, this being at a temperature which generally ranges from 300 to 800xc2x0 C. The object of such an operation is not only to render the precious metals still more reactive, but also the cerium present in the spent catalyst, by removing any oxide layer which may be coating said precious metals and/or by reducing the cerium initially present in an oxidation state of +4 to cerium in an oxidation state of +3, which has been determined to be more easily leachable.
According to an essential first characteristic of the process of the invention, the catalyst powder is next intimately admixed with sulfuric acid. The amount of sulfuric acid, expressed as pure H2SO4, employed in this stage advantageously ranges from approximately 0.4 to 5 times the weight of the catalyst powder to be treated, and preferably ranges from 0.5 to 2 times this weight, and still more preferably from 0.8 to 1.5 times such weight. It will be appreciated that the use of quantities of acid which are larger than approximately 5 times the weight of catalyst as indicated above, while theoretically possible, does not present any particular advantage or significantly improved results. Thus, no benefit is seen from a practical viewpoint.
In most cases the sulfuric acid is used in the form of a solution, generally aqueous, the latter preferably being relatively concentrated, namely, its sulfuric acid content is more than 50% by weight and, still more preferably, more than 80% by weight. It is of course also possible to use sulfuric acids diluted in other solvents, or to use pure sulfuric acid. It too will be appreciated that the purity and/or the quality of the acid introduced is not really important and, in particular, it is possible to use residual sulfuric acids or acids originating, for example, from the roasting of sulfide ores, especially blende ores. This adds considerably to the advantage and the economics of the process of the invention.
After the above products have been mixed until a more or less solid composition has been formed and obtained, which in most cases has the appearance of a greyish paste, calcining of the resulting physical mixture is next carried out, generally in air.
This calcination is typically conducted at a temperature of from 150xc2x0 C. to 450xc2x0 C., in particular from 200xc2x0 C. to 400xc2x0 C., this being for a sufficient time (which depends in particular on the quantity of pasted catalyst) to ultimately produce a brittle whitish solid, designated xe2x80x9cbaked sulfates.xe2x80x9d
During this stage, deemed sulfuric pasting, chemical reactions are initiated and take place in the mixture based on catalyst and sulfuric acid. In particular, on the one hand, the release of acidic white fumes originating from the decomposition of sulfuric acid and, on the other, the conversion of the chemical species present into sulfates are observed.
It will here be appreciated that a proportion of the sulfuric acid which is employed for the pasting can be recycled into the process by treating the acidic white fumes which are released during the formation of the baked sulfates.
In accordance with another essential characteristic of the process of according to the invention, the precious metals and the cerium which are present in the composition originating from the sulfuric pasting stage must next be leached.
As indicated above, this leaching can then be carried out according to two alternative embodiments.
According to the first alternative embodiment, the baked sulfates are first leached with an acidic solution. The first leaching, which is preferably conducted at a temperature of at least 50xc2x0 C., is next followed by a filtration which thus makes it possible to obtain, on the one hand, a first filtrate and, on the other, a first solid residue. This first filtrate (acidic sulfate solution) then contains most of the cerium (at least 70%) and of the rhodium (approximately two thirds) which were initially present in the catalyst, but is, one the other hand, substantially free from any platinum values. Then, in a second stage, a leaching of said first residue is carried out, this time using a solution containing, on the one hand, chloride ions and, on the other, preferably an oxidizing agent.
After the digestion, the resulting suspension is filtered and thus produces a second filtrate and a second solid residue. This second leaching, which is preferably carried out with the solution boiling, makes it possible, in turn, to dissolve and to recover in the second filtrate (acidic chlorosulfates solution) most of the platinum which was present in the starting catalyst, as well as the remaining quantities of cerium and rhodium which were not leached out during the first operation. It too will be appreciated that the first or second solid residues referred to above may be washed with water, or, with an acidic aqueous solution, to recover the impregnating liquors and possibly the small amounts of platinum or rhodium which are still adsorbed. In this case, the aqueous washes are then mixed with the first and second filtrates respectively. Finally, it will also be appreciated that the second residue may optionally be recycled, either to the pasting stage, or to the first leaching stage, or to the second leaching stage.
According to a second alternative embodiment of the process of the invention, which is preferred to the first, the baked sulfates are leached with a single acidic solution containing chloride ions as well as, preferably, an oxidizing agent. This leaching is preferably carried out at a temperature of at least 50xc2x0 C. and still more preferably with the solution boiling.
In this case, after leaching, a suspension is obtained which is next filtered, and this provides, on the one hand, a filtrate which then contains all of the precious metals and the cerium which were present in the starting catalyst, and a solid digestion residue.
This second alternative embodiment presents the advantage, when compared with the first, of permitting the precious metals and the cerium to be simultaneously dissolved in a single stage and with very high efficiencies.
It should be noted, furthermore, that all of the considerations in the context of the first alternative embodiment in respect of the possibilities of washing and/or recycling the digestion residue are similarly applicable to the second alternative embodiment.
The following general comments are made concerning either of the alternative embodiments described above.
Exemplary oxidizing agents that can be employed, whether alone or in admixture, according to the present invention include hydrogen peroxide, nitric acid, chlorine, sodium persulfate and oxygen. The function of this oxidizing agent is to oxidize the platinum, the palladium and the rhodium and to render these species more soluble during the leaching stages. The amounts of oxidizing agents which are introduced, therefore, generally correspond to the stoichiometric amounts in relation to the total amount of the oxidizable platinum, palladium and rhodium values which are present in the mixture.
The chloride ions, in turn, may be introduced in the form of hydrochloric acid, gaseous chlorine or salts, for example alkali and/or alkaline earth metal chlorides such as sodium chloride, potassium chloride, calcium chloride, ammonium chloride and the like. The function of these chloride ions according to the invention is to permit complexing of the platinum and of a proportion of the rhodium. To this end, the amounts of chloride ions to be used correspond at least to the stoichiometry necessary in relation to the oxidized platinum, palladium and rhodium species present in the mixture.
Finally, the necessary acidity of the leaching solutions to be used in accordance with the invention may be introduced using many acids, in particular strong inorganic acids such a sulfuric acid, nitric acid, or hydrochloric acid. The concentrations of acid in the initial leaching solutions are then fixed such that the pH of the solutions obtained directly after the leaching stage (second alternative embodiment) or stages (first alternative embodiment) is still lower than or equal to 2 and preferably lower than or equal to 1. Indeed, with final pH values higher than 2, problems of insolubility of certain species in the mixture can then arise.
It will be appreciated that the same single compound can therefore play a number of roles according to the invention, namely, to provide at the same time the acidity and/or the chloride ions and/or the oxidizing nature which are necessary for its use. This is the case, for example, with hydrochloric acid HCl (H+ acidity and Clxe2x88x92 ions), with nitric acid (H+ acidity and oxidizing nature) and with chlorine (Clxe2x88x92 ions and oxidizing nature).
A particular and advantageous attribute of the second alternative embodiment of the process according to the invention as described above (a single leaching stage), is when the sole leaching solution employed in stage (iv) is a sulfuric acid solution (introduction of H+ ions) containing alkali metal chlorides such as, for example, sodium, potassium or ammonium chlorides (introduction of chloride ions). In this particular case, and assuming that the initial composition being processed contains cerium, the simultaneous presence of, on the one hand, sulfate ions and, on the other, alkali metal ions (NA+, K+, NH4+, etc.) in the processing solution results in the formation of cerium alkali metal double sulfates which are compounds that are insoluble under the conditions of processing. These compounds, therefore, do not go into solution with the precious metals and thus remain, after filtration, with the solid digestion residue. This constitutes a particularly advantageous and efficient means for carrying out a recovery which is selective between, on the one hand, the precious metals and, on the other, cerium, while remaining within the scope of the second alternative embodiment of the process of the invention. The cerium can subsequently be recovered from the solid residue by any technique which is per se known to this art, for example by selective redissolving.
The precise metals, and optionally the cerium which are present in the solutions originating from the leaching stages described above can subsequently be recovered from the latter by any appropriate technique which is per se known to this art, in particular by precipitation, cementation, liquid/liquid extraction or else ion exchange and then optionally be finally purified. The precious metals and/or cerium which are thus obtained can then be used again for the manufacture of new catalysts.